Technological development and increased demand for mobile equipment have led to a rapid increase in the demand for secondary batteries as energy sources. Among these secondary batteries, lithium secondary batteries having high energy density and voltage, long lifespan and low self-discharge are commercially available and widely used.
In addition, increased interest in environmental issues has brought about a great deal of research associated with electric vehicles (EVs) and hybrid electric vehicles (HEVs) as substitutes for vehicles using fossil fuels such as gasoline vehicles and diesel vehicles which are major causes of air pollution. These electric vehicles generally use nickel metal hydride (Ni—MH) secondary batteries as power sources of electric vehicles (EVs), hybrid electric vehicles (HEVs) and the like. However, a great deal of study associated with use of lithium secondary batteries with high energy density and discharge voltage is currently underway and some are commercially available.
In particular, lithium secondary batteries used for electric vehicles should have high energy density, exhibit great power within a short time and be used for 10 years or longer under harsh conditions in which charge and discharge based on high current are repeated within a short time, thus requiring considerably superior stability and long lifespan, as compared to conventional small lithium secondary batteries.
Conventional lithium secondary batteries generally utilize a lithium cobalt composite oxide having a layered structure for a cathode and a graphite-based material for an anode. However, such lithium cobalt composite oxide is disadvantageously unsuitable for electric vehicles in terms of presence of extremely expensive cobalt as a main element and low safety. In order to solve these disadvantages, materials such as Li(NixMnyCozO2) (x+y+z=1) are used.
In order to secure structural stability of such a layer-structure cathode active material, many researchers have studied cathode active materials with a layered structure containing Li2MnO3. The cathode active materials with a layered structure containing Li2MnO3 is characterized in that Li is contained in a general transition metal layer made of LiMO2 (M: transition metal) and they have super lattice peaks caused by the Li2MnO3 structure. Such a material advantageously contains a great amount of Mn and is thus considerably cheap and exhibits considerably high capacity at a high voltage. The material has a flat voltage region of 4.4 to 4.6V. After activation occurs in the flat region, capacity increases. This increase in capacity is known to be caused by deintercalation of Li from the transition metal layer due to generation of oxygen, but is still controversial.
Capacity increases after the activation region, but rate characteristics are clearly rapidly deteriorated. Due to these characteristics, this material is practically inapplicable to batteries at present.
In order to solve these problems in the related art, a method in which the active material is coated with particles after it is synthesized, has been attempted, but this method disadvantageously causes an increase in preparation cost. Furthermore, this method uses a post-treatment manner and does not contribute to variation and improvement in substantial internal structure, most structural variation is caused by formation process of crystalline at a high temperature of the synthesis process.